This invention relates generally to erosion control of bluffs, hillsides and the like and, more particularly, pertains to a system and method for stabilizing the bluff or hillside by controlling the internal water drainage so as to prevent landslides or similar land mass slumping along the face or slope of the bluff or hillside.
The erosion of bluffs lying adjacent a lake or ocean has constituted a long and serious environmental problem. These bluffs are subject to the action of the waves occasionally pounding against them which erode away the base of the bluff leaving the bluff with an extremely steep face. This steep front face is very unstable, and, in some instances, is not able to support the weight of the dirt, sand and the like lying above it. Consequently, landslides or slumping may occur which can endanger man-made structures as well as trees and bushes on top of the bluff.
These bluffs as well as hillsides, such as those which slope down to a highway, are also subject, on a larger scale, to the action of forces acting within as caused by groundwater and water from spray, rain or snow melt which matriculates to various layers running through and exiting out the face of the bluffs or hillsides. If water saturation becomes excessively high, the outwardly exerting forces cause collapse of the steep face of the bluff or hillside, sometimes along an appreciable length thereof.
One prior art attempt to mitigate degradation of a hillside is disclosed in U.S. Pat. No. 4,714,376 issued Dec. 22, 1987 to Jenab. In this patent, a perforated drain pipe is placed within a hillside substantially parallel to the face of the hillside at a location on or above an impermeable layer. Jenab is thus primarily concerned with control of water drainage at the bottom of the hillside and is limited to boring holes, construction of a concrete apron and placement of a drain pipe at locations close to the face of the hillside near its bottom edge where a bulldozer is used to facilitate installation. While the ""376 patent discloses the use of another drain pipe at a higher level of the hillside, it is only possible that the location of such pipe is within the reaching limits of the bulldozer leading one to believe it must be near the bottom of the hillside. Installation of Jenab""s drain pipes through the front face of the hillside would thus appear to contribute to the collapse of the front face of the hillside, the very problem which the present invention seeks to prevent.
Another attempt to rectify bluff erosion by controlling internal water drainage has resulted in installing horizontal wick drains through the face of the bluff at angles generally perpendicular thereto. These installations are effected by forming a series of bores in upper elevations of the bluff using drilling equipment mounted on a support device suspended outside the face of the bluff. While such installations have been generally satisfactory for their intended purpose, they have not proven to be as efficient as desired in regulating bluff drainage. They also can exacerbate the problem because they conduct water and turn many drilled holes perpendicular to the bluff which adds water to the mass between the bluff shear plane and the bluff face.
Accordingly, it is desirable to provide a differently styled system and method for preventing bluff or hillside erosion which is more efficient in stabilizing the face of the bluff or hillside by controlling the water seepage therein.
It is a general object of the present invention to provide a novel and improved system which alters the natural drainage of water from the bluff so as to substantially preserve the formation thereof.
It is also an object of the present invention to strategically position a horizontal wick drain parallel to the crest or toe of the bluff in a water saturated layer or layers running therethrough.
It is a further object of the present invention to provide a unique method of installing a horizontal wick drain in a bluff using conventional drilling equipment.
It is an additional object of the present invention to provide for channeling water down the face of the bluff to a beach level using collection pipes attached to wick drain end points.
It is another object of the present invention to provide a bluff erosion protection system which eliminates the need for insertion of drainage structure by drilling or excavating the face of the bluff or hillside.
A still further object of the present invention is to provide a dewatering system for a bluff which does not require supplemental physical structure at the base or along the slope of the bluff.
In one aspect of the invention, a system is provided for preventing erosion of a bluff or hillside having a face extending between a toe at a bottom thereof and a crest at a top surface thereof, and a failure shear plane. The bluff or hillside is subject to excess water accumulation contributing to land mass degradation thereof. The system is improved by at least one horizontal wick drain positioned behind the failure shear plane in a water saturated zone substantially parallel to the crest of the bluff or hillside for the purpose of controlling drainage of the excess water accumulation. The horizontal wick drain is installed from the top surface of the bluff or hillside so as not to impair the structural integrity or cause land mass degradation of the face thereof. The wick drain has a proximal end located at the top surface of the bluff or hillside, and a distal end located in the water saturated zone at the face of the bluff or hillside. The distal end of the wick drain curves to exit at the bluff face and is connected to a solid drain pipe which extends downwardly along the face of the bluff or hillside and terminates in advance of the toe of the bluff or hillside. The wick drain is installed in a hole formed by a horizontal directional boring machine positioned on the top surface of the bluff or hillside.
In another aspect of the invention, in a bluff lying adjacent a body of water and having a shear failure plane and a face extending from a toe at the bottom of the bluff to a crest at the top surface of the bluff, there is provided a drainage system for preventing erosion of the bluff caused by excess water forces acting outwardly towards the face of the bluff. The system includes an elongated bore extending substantially parallel to the crest of the bluff behind the shear failure plane and having a first end at the top surface of the bluff, and a second end exiting through the face of the bluff, the bore being formed from the top surface of the bluff. An elongated horizontal wick drain has a plastic corrugated core surrounded by a filtering fabric and is positioned within the bore, the wick drain further having a proximal end at the first end of the bore and a distal end at the second end of the bore. A collector drain pipe is connected to the distal end of the wick drain for returning excess water accumulated to the wick drain in the adjacent body of water. The bluff includes a water attracting sand layer interposed between a pair of clay layers. The bore and the wick drain are located substantially in the sand layer.
The invention further contemplates a method for preventing erosion in a face of a bluff having accumulated water therein, the face running from a toe of the bluff to a crest of the bluff at a top surface thereof. The method includes the steps of drilling a series of substantially vertical holes into the top surface of the bluff to locate a shear failure plane and water bearing layers in the bluff; advancing a drill string from an out hole at a top surface of the bluff downwardly on a radius, horizontally in a water saturated zone behind the shear failure plane and then outwardly to the face of the bluff; connecting an end of the drill string at the face of the bluff to a proximal end of a horizontal wick drain for channeling water accumulated in the bluff; withdrawing the drill string from the out hole at the top surface of the bluff so as to drag the wick drain through the out hole bringing the proximal end of the wick drain to the top surface of the bluff and a distal end of the wick drain to the face of the bluff; disconnecting the wick drain from the drill string when the proximal end reaches the top surface of the bluff; and attaching an input end of a collection drain pipe to the distal end of the wick drain for directing water accumulated in the wick drain via an outlet end away from the face of the bluff.
The step of advancing the drill string in the horizontal hole is performed using a horizontal directional boring machine located at the top surface of the bluff. The steps of connecting an end of the drill string to the proximal end of a horizontal wick drain, and the step of attaching the collector drain pipe to the distal end of the wick drain are performed by a worker suspended in a cherry picker positioned on the top surface of the bluff. All of the steps avoid excavation and drilling of the face of the bluff.